1. Field of Invention
The present invention relates to the field of flexible printed wiring boards, particularly to the field of flexible printed wiring boards of multilayer structure.
2. Description of Related Art
Recently, flexible wiring boards of multilayer structure are used in many electronic circuits.
As an example, a process for manufacturing a multilayer flexible printed wiring board is explained. Referring to FIG. 20(a), the reference number 311 represents a copper foil having a thickness of dozens of micrometers.
A polyimide varnish is first applied on this copper foil 311 to form a base film 312 consisting of a polyimide film (FIG. 20(b)). Then, a resist layer 313 is formed on base film 312 (FIG. 20(c)), and resist layer 313 is patterned via photographic processes. The reference number 331 in FIG. 20(d) represents an opening in resist layer 313, and base film 312 is exposed at the bottom of this opening 331.
Then, the part of base film 312 exposed at the bottom of opening 331 is etched off (FIG. 20(e)). Then, resist layer 313 is removed to give a patterned base film 312 (FIG. 20(f)).
In FIG. 21(g), base film 312 is inverted with copper foil 311 upward. A masking film 317 is applied on base film 312 (FIG. 21(h)), and a resist layer 315 is formed on copper foil 311 (FIG. 21(i)).
Then, resist layer 315 is patterned via exposure and development processes. The reference number 332 in FIG. 21(j)) represents an opening formed by patterning in resist layer 315. Copper foil 311 is exposed at the bottom of this opening 332.
Then, copper foil 311 at the bottom of opening 332 is etched to pattern copper foil 311 into a first wiring layer 316 (FIG. 21(k)). The reference number 333 represents the part from which copper foil 311 has been removed and an opening segmenting first wiring layer 316. The top of base film 312 is exposed at the bottom of opening 333. Resist layer 315 is removed (FIG. 21(l)) and a polyimide varnish is applied on the top of first wiring layer 316 so that the polyimide varnish flows into opening 333 in first wiring layer 316 to form a cover film 318 consisting of a polyimide film having a flat surface. A resist layer 319 is formed on the top of cover film 318 (FIG. 22(n)) and resist layer 319 is patterned via exposure and development processes.
The reference number 334 in FIG. 22(o) represents an opening formed by patterning in resist layer 319. Cover film 318 is exposed at the bottom of this opening 334.
Then, the part of cover film 318 located at the bottom of opening 334 is etched off with a metallic etching solution to pattern cover film 318 so that first wiring layer 316 is exposed at the bottom of the opening 334. The etching solution used here is selected not to etch first wiring layer 316.
Finally, resist layer 319 is removed and followed by heat treatment to imidate base film 312 and cover film 318, whereby a first single-wiring layer board piece 310 is obtained (FIG. 22(q)).
Thus obtained first single-wiring layer board piece 310 comprises first wiring layer 316, patterned base film 312 provided on one side of first wiring layer 316 and patterned cover film 318 provided on the opposite side of first wiring layer 316. Opening 333 in first wiring layer 316 is filled with cover film 318.
The reference number 380 in FIG. 23(a) represents a second single-wiring layer board piece to be laminated to first single-wiring layer board piece 310. This second single-wiring layer board piece 380 comprises a base film 381 consisting of a polyimide film, a second wiring layer 386 provided on said base film 381 and a cover film 382 provided on said second wiring layer 386.
Said second wiring layer 386 consists of a patterned copper foil and said cover film 382 consists of a polyimide film.
Second single-wiring layer board piece 380 has a plurality of bumps 384 connected to second wiring layer 386 at the bottoms and projecting from cover film 382 at the tops.
First single-wiring layer board piece 310 is opposed to the plane of second single-wiring layer board piece 380 from which the tops of bumps 384 project in parallel thereto, and bumps 384 are aligned with openings 331 in base film 312 to bring bumps 384 into contact with the surface of first wiring layer 316, whereby first and second wiring layers 316 and 386 are connected via bumps 384.
If either one of two cover films 312, 382 includes of a thermoplastic resin having the property of developing adhesiveness upon heating, first and second single-wiring layer board pieces 310, 380 can be bonded together by heating them while bumps 384 are in contact with the surface of first wiring layer 316. The reference number 351 in FIG. 23(b) represents a multilayer wiring board comprising first and second single-wiring layer board pieces 310, 380 bonded together.
The process for forming an opening by patterning a polyimide film by etching as described above provides finer openings than laser etching or drilling so that it is widely used in the manufacture of high-density multilayer flexible wiring boards in which openings should be provided with narrow gaps.
However, the etching process using an alkali solution as described above involves complex control of the temperature or state of the solution. Particularly when etching conditions are insufficiently controlled, variation may occur in the size of openings formed in polyimide.
Moreover, the use of a resist layer consisting of a photosensitive film for forming an opening adds production costs.
An object of the present invention is to simplify the complex conventional process for manufacturing a multilayer wiring board as described above and to provide a single-layer flexible wiring board suitable for preparing a multilayer flexible wiring board, the resulting multilayer flexible wiring board, a process for manufacturing a multilayer flexible wiring board and an ultrasonic manufacturing apparatus suitable for use in the manufacturing process.